The present invention relates to a method for producing a silicone elastomer based insulating materials containing additives of max. 55 percent by weight, particularly resistant to damages done by birds, and suitable for being applied to medium-and high-voltage outdoor insulators. The invention relates, further, to a method and equipment for manufacturing outdoor insulators having fibre-reinforced cores provided with sheds for increasing the creepage distance (leakage path) along the insulator surface.
It is known that a high-voltage insulator body of solid silicone rubber attaining its cross-linked (referred to as HTV) high temperature vulcanized (in the following) structure, at elevated temperatures can only be produced in several steps, as stated, e.g., in West-German Patent of Rosenthal Technic AG. In the manufacturing process specified in this patent, first, a HTV silicone rubber layer is extruded over a glass-fibre-reinforced rod carrying the mechanical load and, then, cured. In the second step, the insulator sheds of required size are produced, one after the other, also cured from HTV silicone, by the traditional pressing process commonly used in rubber industry, then, as third step, the sheds thus obtained are pushed over the already coated rod and bonded to it with a special adhesive.
The method described also confirms that insulator bodies of complex shape and large size cannot be produced from HTV silicone rubber by conventional pressing technology. This is due to the fact that for the continuity of insulator material (i.e.) its freedom from blisters, demanded from the point of view of the electric power industry, extremely high specific extruding pressure and, correspondingly, specially shaped tools and extruding equipment are required. Beyond all this, in the course of extrusion, the mechanically loaded glass-fibre reinforced core (generally, having the shape of a rod or tube) tends to suffer cracks, destructions or deformations. In addition, the method proposed by the West-German Patent is rather labor- and energy-intensive, due to its customary characteristics. Another problem lies in the mechanically and electrically weak points developing on the bonded surfaces of silicone rubber components. Consequently, no economical way of producing such insulator bodies has been found yet. Neither the so-called liquid silicone rubber (LSR) suitable for being processed by injection moulding has brought about any revolutionary changes in the production of such insulating bodies, since its appearance during the second half of the 70's. The reason lies partly in the difficulty associated with the extraordinarily high tool-closing forces required in the production of bulky or larger-size bodies, the processing technology being akin to that used in processing thermoplastics. Consequently, the LSR-type materials have found their application in the production of small-size technical and sanitary articles (O-rings, vial stoppers, soothers and teats, etc.) Moreover, the worm feeder type moulding machines, having become wide-spread in the meantime are not suitable for blister-free, safe manufacture of larger-size products.
In British Pat. No. 1292 278 a composite insulator is described in which a glass-fibre-reinforced rod for taking up mechanical loads is located concentrically and the surface of which is coated with a material resisting to creepage currents, and over which prefabricated sheds, shrinking under the effect of heat, are pushed. The sheds are then glued to the rod by means of a coating made of a heat-melted compound. A great disadvantage of the method proposed by the British patent is the shrinkage of the structural material under the effect of heat. This is due to the very low shrinkage stress of compounds partially mouldable as thermoplastic materials, this shrinkage stress being so low that practically no compressive force develops between the mantle of the supporting rod and the shed pushed over it, whereby small voids and cracks remain in the filling material of interstices in which the water entering by diffusion tends to condense, leading to electric breakdowns. All this applies also to the coating of the rod consisting of the same material and fixed in the same way as the shed.
Another method has been described in a West-German Laid-Open Application No. 22 54 468, where the mutually overlapping sheds are made of butyl rubber and are fixed in the longitudinal axis of the concentrically arranged supporting rod. The sheds are prefabricated and pushed over said supporting rod to which silicone grease is applied previously. The deficiency of this type lies in the poor resistance of butyl rubber to oxidation in outdoor applications, resulting in an insufficient leakage resistance. Neither the proposed silicone grease as intermediate layer exhibits an acceptable resistance to conditions prevailing outdoors. In the electric field built up across the butyl rubber, the silicone grease suffers chemical decomposition, in the course of which conductive components are formed, resulting in electric breakdown between the sheds and supporting rod.
When examining the insulation proposed by the present invention, the following can be stated concerning the composition of the liquid silicone elastomer with its cross-linked structure developing according to the addition mechanism well-known from literature (e.g. see U.S. specifications of Nos. 3,697,473, 3,884,866, 4,162,243 and 4,427,801), the components are as follows:
within the triorgano-siloxy terminal group a poly-diorgano siloxane containing a vinyl group, termed in some cases also as vinyl-functional polysiloxane;
organohydrogen-siloxane compounds (H-functional polysiloxane);
a platinum-containing catalyst,
further, in some cases, an inhibitor, a pigment and a filler.
The reaction bringing about the cross-linking structure and the addition of loose H of organo-hydrogen-siloxane to the vinyl-group of the poly-disorganosiloxane, which is catalyzed, as commonly known, by compounds of platinum.
The characteristic properties of products made of liquid silicone rubber are rather modest as compared to traditional solid silicone rubbers attaining their cross-linked structure in hot state by means of a peroxide treatment or by way of addition (:J. Karger-Kocsis: Muszaki Gazdasagi Tajekoztato25, 1565 (1984):) by introducing, as filler, colloidal silicic acid, surface- treated ("silylized") by silyle compounds (see, e.g. U.S. Pat. No. 3,122,516 and West-German Patent No. 2953 252) or by suitably selecting the quality of vinyl-functional polysiloxane (U.S. Pat. Nos. 3,671,480 and 3,697,473) and West-German Patent No. 2918313) or possibly, by combining the two compounds U.S. Pat. 4,427,801 and West-German Patent 2,918,313).
Liquid silicone rubbers have not been used for manufacturing electric insulators, because
on the one hand, no satisfactory results have been achieved so far in producing such large-size bodies from these materials
on the other hand, the damage done by birds could not be avoided even with types of increased spalling resistance.
Any improvement of physico-mechanical characteristics of liquid silicone rubbers could be achieved almost exclusively by the use of colloidal silicic acid (pyrogenic SiO.sub.2) in its surface-treated form. However, this would increases further the anyway relatively high viscosity (5.times.10.sup.5 to 1.times.10.sup.6 mPas) of liquid silicone rubber, making its processing into larger-size bodies impossible. According to relevant specifications, insulators may be made of relatively soft (40 to 60 Shore A) silicone rubber, yet their reliable service is often jeopardized by birds--mainly by crows--chipping off pieces from the insulators, and this cannot be prevented by increasing the spalling resistance. Thus, it is not a mere chance that the traditional material of outdoor electric insulators is epoxy resin. Although the process for preparing epoxy-modified silicones is known from literature (e.g. No. 4,394,013 U.S. patent) yet, due to the complexity of their manufacture, they have not gained ground in that field of application.
Among the requirements to be fulfilled by the material of insulators that of reduced combustibility often appears. In the case of silicone elastomers, for this purpose, various metallic compounds, as MgO (West-German Patent 308608 ZnO and MgO West-German Patent 2257915) and, in addition to the former, aluminium- and tin-oxides (West-German Patent 2308595) cerium salts (U.S. Pat. Nos. 3,264,382 and 3,884,950, as well as U.K. Pat. No. 1,299,687, titanium oxide and iron oxide West-German patent 2617434, platinum compounds (West-German patent 2,849,228) or, possibly, a mixture of organic bromine compounds and fillers are applied. The use of the above compounds in the case of silicone rubbers containing an addition-type cross-linked platinum catalyser may be hazardous, since by many of the enumerated compounds the platinum catalyser is contaminated and inactivated, impeding the process of curing (West-German patent 2849228, p. 3, lines 24 to 34).
According to the present state of the art, no method is known by which, without considerably influencing the viscosity (curing) of liquid silicone rubber, the possibility could be opened toward the production of insulators protected against the damaging effects done by birds and, at the same time, possessing the required property of incombustibility.